Proximity sensors such as ultrasonic sensors have been widely used to detect distance to objects. In particular, ultrasonic sensors are typically configured to generate ultrasonic signals with an ultrasonic transducer and to receive the echo signals reflected back by the objects. By calculating the time interval between sending the ultrasonic signal and receiving the echo signal, the distance to an object can be determined based on the propagation speed of sound through the propagation medium such as air.
Traditionally, the application of the ultrasonic sensors is limited by the existence of the blind zone, which is caused by residual mechanical vibration of the ultrasonic transducer. Ultrasonic transducers are typically configured to generate ultrasonic signals by high frequency vibrations or resonance caused by an excitation signal. For example, a pulse of electrical energy can cause a piezoelectric transducer to vibrate at a given frequency due to piezoelectricity, thereby generating an ultrasonic sound wave. The echo of the transmitted ultrasonic signal as reflected by an object can then be detected and evaluated to determine a distance to the object. However, once the excitation signal (e.g., electrical signal) is removed, the vibration of the transducer usually does not stop immediately. Rather, due to elasticity, the transducer typically continues to vibrate for a period of time, albeit in a dampening fashion. Such residual vibration or reverberation can be detected by the ultrasonic sensor. Reverberation signals can obscure the detection of echo signals. The blind zone is the area surrounding the ultrasonic transducer in which echo signals cannot be reliably detected as distinguished from reverberation signals.
Existing methods attempt to solve the problem of blind zone using either a software approach or a mechanical approach. Under the software approach, the detection of ultrasonic signals is disabled during the time period corresponding to the blind zone so as to avoid detecting the reverberation signals as echo signals. However, the software approach merely avoids but does not reduce or remove the blind zone. That is, objects located within the blind zone still cannot be reliably detected. Using a mechanical approach, the receiver probe of the ultrasonic sensor can be padded or otherwise protected using physical barriers. While this approach can reduce or remove the blind zone by reducing the amplitude of the reverberation signals reaching the receiver probe, the complexity and cost of manufacturing is likely to increase.